Measurement of Serum Steroid and Gonadotropin Levels and Uterine and Ovarian Variables throughout 4 Day and 5 Day Estrous Cycles in the Rat1

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Abstract
The present study was undertaken to characterize hormonal and morphological events in 4 day and 5 day rat cycles, in order to test 3 different theories concerning the etiological factors altering follicular phase length in mammals. Three rats were killed at each 2 h interval throughout each cycle type. In addition to organ weight measurements, serum levels of estradiol [E2], progesterone [P], 20.alpha.-hydroxypreg-4-en-3-one, LH [lutropin], FSH [follitropin] and prolactin were assessed in terminal samples. The beginning of uterine intraluminal fluid accumulation coincided with the decline in P levels seen during diestrus, rather than with the rise in estrogen and was delayed 12 h in the 5 day cycle from the time of last estrus. The retention of this fluid during the evening of proestus was 6 h longer in the 4 day cycling rat. Ovulation occurred earlier in the 5 day cycling rats than in those with a 4 day cycle. No significant differences in the gonadotropins or 20.alpha.-hydroxypreg-4-en-3-one were seen when comparing the 2 types of cycles. E2 rose during the night of diestrus I (the same time in the cycle) and was identical in both cycle types. P was not released on proestrus prior to the LH surge in either type of cycle. However, P values were higher for a longer period of time throughout diestrus I in 5 day cycles. The 5 day cycle is due to a prolonged P secretion during the metestrous and diestrous stages of the cycle.